This invention relates to an aqueous pigment ink set for ink jet recording. The aqueous pigment ink set of the invention effectively prevents blur or color bleeding, particularly at the boundary of different colors in color printing, and further prevents feathering, thus enabling to provide printed images with a high quality.
In the case of printing a recording medium, called plain paper, according to an ink jet recording method using an aqueous pigment ink composition containing a pigment as a colorant, quick-drying properties and high color-developing properties are required. Thus, it is common to add a penetrating agent or a surfactant to the aqueous pigment-based ink compositions for the ink jet recording method. That is, addition of the penetrating agent serves to improve penetrating properties of the compositions, and addition of the surfactant serves to improve wetting properties of the compositions, thus changing the ink properties.
On the other hand, plain paper to be used as recording medium has varying surface state due to a difference in kind of a sizing agent, a difference in composition of pulp constituting the paper, or a difference in paper-making process. It has commonly been known that, due to the difference in the surface state of paper and the change in ink properties, printed letters or images might undergo color blur, a phenomenon that an ink spreads along the pulp fibers constituting the paper to generate beard-like streaks (feathering), or a phenomenon in a region where two or more color ink compositions are superposed one over the other, that the ink compositions are mutually mingled to make the boundary unclear (color bleeding).
As means for preventing the color blur, feathering or color bleeding, there have been made various approaches with respect to ink compositions.
For example, JP-A-7-145336 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes to reduce bleeding by the reaction between an anionic printing solution containing a polymer and a cationic printing solution. Addition of the polymer, however, largely increases viscosity of the ink even when the polymer is added only in a small amount, and hence it is difficult to use the ink for the ink jet recording method in which an ink composition having a viscosity in a comparatively low region (1 to 10 mPa.s) is used. In addition, when the addition amount of the polymer is smaller, the intended bleed-suppressing effect is insufficient, thus the object not being attained.
JP-A-7-150090 proposes a combination of a hydroxycarboxylic acid salt or a hydroxydicarboxylic acid salt and a particular anionic surfactant salt. However, existence of a free carboxylic acid in an aqueous ink involves the problem that ejection stability is deteriorated, causing dot curving problem of ink droplets.
JP-A-8-319442 proposes a technique of adding inorganic fine particles such as colloidal silica or synthetic zeolite to functionalize (specifically make thixotropic) rheology properties of an ink. However, the addition of the inorganic fine particles decreases fluidity of an ink, and therefore causes such problems as dot curving problem of ink droplets and clogging due to deposition of the particles at the nozzle opening. Additionally, even in the case of using organic fine particles or an organic/inorganic hybrid material, the same problem tends to occur where the concentration of solids in an ink is high.
In addition, JP-A-10-6493, JP-A-11-323225, JP-A-11-342635, JP-A-11-343440 and JP-A-11-349878 propose a technique of utilizing anion-cation electrostatic mutual action. In the technique described in these publications, however, there is involved a trouble that a coagulation reaction takes place due to the anion-cation electrostatic mutual action on a nozzle plate upon rubbing a nozzle plate by a cleaning blade in the case of cleaning a printing head, resulting in clogging of the nozzle. In order to avoid this trouble, it is necessary to separate the printing head into a head for an anionic ink and a head for a cationic ink and provide each of the heads with a separate cleaning mechanism, which imposes a mechanical burden and causes an increase in production cost.
In addition to the techniques of adding additives to an ink composition as described above, there has been proposed a technique of preventing color blur, feathering or color bleeding by using an ink composition and a reactant in combination.
For example, JP-A-9-239964 proposes a technique of using a reactant containing a coordination compound of a multi-valent metal. This is a technique of utilizing a coagulation reaction to be caused between a multi-valent metal ion (cation) and a dye having an anionic functional group. However, the multi-valent metal ion to be used in this technique as a reactant is an essentially unstable compound liable to separate out, and hence a reaction solution is previously prepared in which this ion is stably dissolved by forming its complex with a coordination compound and, upon printing, this reaction solution is brought into contact with an ink composition. Therefore, in case where the coordination state is destroyed by any reason, a metal compound separates out in the reaction solution and clogs piping or nozzles as insolubles.
Further, in the technique of using the aforesaid reactant, it is necessary to separate the printing head into a head for a reaction solution and a head for an ink composition and separately provide a cleaning mechanism for each of the heads, which imposes a burden of apparatus and causes an increase in production cost. In addition, it has been known that, in the case of conducting printing, reaction solution droplets are ejected onto a recording medium such as paper in addition to ink droplets, and hence an amount of water to be given to the recording medium is increased so much that there are involved problems that a prolonged drying time is required and that deformation of the recording medium, i.e., wrinkling or curling, takes place. In general, it is also known that, even when a highly reactive reaction is employed, an excess amount of water given to the recording medium would make it difficult to prevent color blur, feathering or color bleeding.
In the full color ink jet printing method using an aqueous pigment-based ink composition set, the color blur, feathering or color bleeding as described hereinbefore is observed seriously in the case of recording letters with a color having a low lightness such as black against the background of a color having a high lightness such as yellow at the boundary therebetween. Means for effectively preventing color blur, feathering or color bleeding under the condition where color blur, feathering or color bleeding becomes most conspicuous as described above have not so far been known.
Accordingly, an object of the present invention is to provide an aqueous pigment-based ink composition set for a color ink jet recording apparatus, which effectively prevents color blur, feathering or color bleeding even at the boundary between a color having a high lightness such as yellow and a color having a low lightness such as black, thereby enabling to produce recorded products with a high quality.
Other objects and effects of the present invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing:
an ink set comprising at least two aqueous ink compositions each containing a dispersed pigment and a surfactant, said ink compositions including an ink composition having the lowest lightness and an ink composition having the highest lightness;
wherein (1) said ink composition having the lowest lightness has a higher surface tension than those of other ink compositions;
wherein (2) said ink composition having the highest lightness has a lower surface tension than those of other ink compositions; and
wherein (3) the dispersed pigment contained in said ink composition having the lowest lightness and the dispersed pigment contained in said ink composition having the highest lightness are (3a) both anionic dispersed pigments or (3b) both cationic dispersed pigments,
when condition (3a) is satisfied,
(3a-i) said ink composition having the lowest lightness contains an anionic surfactant, and said ink composition having the highest lightness contains an amphoteric surfactant, or
(3a-ii) said ink composition having the lowest lightness contains an amphoteric surfactant, and said ink composition having the highest lightness contains an anionic surfactant, and
when condition (3b) is satisfied,
(3b-i) said ink composition having the lowest lightness contains a cationic surfactant, and said ink composition having the highest lightness contains an amphoteric surfactant, or
(3b-ii) said ink composition having the lowest lightness contains an amphoteric surfactant, and said ink composition having the highest lightness contains a cationic surfactant.
In a preferred embodiment, the ink composition having the lowest lightness is a black ink composition and/or the ink composition having the highest lightness is a yellow ink composition.
In another preferred embodiment, the dispersed pigment in the ink composition having the lowest lightness and the dispersed pigment in the ink composition having the highest lightness are both anionic dispersed pigments.
In a still other preferred embodiment, the anionic surfactant has a sulfonic acid group and/or the amphoteric surfactant has a sulfonic acid group.
In a still other preferred embodiment, the ink compositions, other than said ink composition having the highest lightness and said ink composition having the lowest lightness, each contain a nonionic surfactant.
In a still other preferred embodiment, the ink composition having the highest lightness has a surface tension of 20 mN/m or higher, and the ink composition having the lowest lightness has a surface tension of 50 mN/m or lower.
In a still other preferred embodiment, the ink set comprises a yellow aqueous ink composition, a cyan aqueous ink composition, a magenta aqueous ink composition and a black aqueous ink composition. In this case, it is further preferred that the yellow, cyan, magenta and black aqueous ink compositions have surface tensions satisfying the following relational expressions (1) and (2):
xcex3k greater than xcex3c greater than xcex3yxe2x80x83xe2x80x83(1)
xcex3k greater than xcex3m greater than xcex3yxe2x80x83xe2x80x83(2)
wherein xcex3y, xcex3c, xcex3m and xcex3k represent surface tensions of the yellow, cyan, magenta and black aqueous ink compositions, respectively. In a still further preferred embodiment, the following relational expressions (3) and (4) are satisfied:
20 mN/mxe2x89xa6xcex3yxe2x80x83xe2x80x83(3)
xcex3kxe2x89xa650 mN/mxe2x80x83xe2x80x83(4).